Soil cores were retrieved from 21 saltmarshes (supplemented with data from five other Scottish saltmarshes (Miller et al., 2023)) ( Figure 1 ) between 2018 and 2020. A triple transect sampling strategy (Ladd et al., 2022) was employed where two transects ran perpendicular to the shore, intersecting the different marsh zones (high, mid-low and, where present, pioneer zones), with the third transect running diagonally to the shore intersecting the other transects ( Figure 2 ). At each site, the positioning of the transects were adapted to site specific conditions (e.g., geomorphology, hydrology, vegetation communities). Sampling locations were spaced evenly and in proportion to marsh width, with the coordinates of each coring site recorded by differential global positioning system (dGPS) to an average accuracy of ∼2 cm both in the vertical and horizontal plane. A total of 474 soil cores were collected using a narrow (3 cm diameter) gouge corer which was pushed by hand to either a depth of 1 m, or until a resistant basal layer was reached. Gouge corers assure minimal compaction (Smeaton et al., 2020). The soil profile of each core was described using the Troels-Smith classification scheme (Troels-Smith, 1955) with the depth of transitions recorded. Cores were sub-sampled in the field at depths of 0–2 cm, 4–6 cm, 10–12 cm, 20–22 cm, 30–32 cm and every further 10 cm until 90–92 cm, generating 3,413 sub samples for soil analysis.

Aboveground biomass samples were collected from each marsh, to assess its contribution to the saltmarsh OC stock. The aboveground vegetation was surveyed within 1 m² quadrats at 143 sites across the saltmarshes. Vegetation composition was described following the National Vegetation Classification (NVC) scheme (Rodwell, 2000; Supplementary Table 1 ). Within each quadrat, living vegetation was cut at soil level from an area of 0.125 m² (Harvey et al., 2019) and returned to the laboratory to calculate biomass.

The harvested aboveground biomass (n = 143) samples were oven dried at 60°C for 72 hrs. After drying, the material was weighed and the aboveground biomass calculated for each harvested quadrat (Ford et al., 2019; Harvey et al., 2019; Miller et al., 2023).

The 3,413 soil samples were oven dried at 60°C for 72 hrs. Before and after drying, the samples were weighed for the calculation of wet bulk density, dry bulk density and water content following standard methods (Athy, 1930; Appleby and Oldfield, 1978; Dadey et al., 1992).

The dried soil and biomass samples were milled to a fine powder in preparation for bulk elemental analysis. Then, 50 mg of homogenized sample was weighed into a steel crucible and placed into an Elementar Soli TOC. The Soli TOC utilizes the temperature gradient method (DIN 19539, 2015; Natali et al., 2020; Smeaton et al., 2021) of elemental analysis to quantify OC and inorganic carbon (IC) from a single untreated sample, unlike other methods where acidification steps are required (Verardo et al., 1990; Nieuwenhuize et al., 1994; Harris et al., 2001). This is accomplished through ramped heating of the sample at a rate of 70°C min⁻¹ through sequential furnace temperatures of 600°C and 900°C. The CO₂ evolved at the different temperature ranges represents the fraction of OC (0–600°C) and IC (600–900°C) within the sample. The evolved CO₂ produced within each temperature window is measured by infrared spectrometry and converted to C (%).

The standard deviation of triplicate measurements (n = 200) was OC: 0.10% and IC: 0.21%. Further quality control was assured by the repeat analysis of standard reference material B2290 (silty soil standard from Elemental Microanalysis, United Kingdom); these analyses of standards deviated from the reference value by: OC = 0.09% and IC = 0.14% (n = 420).

The primary data collected within this study were combined with secondary data to support the saltmarsh OC stock estimations. Miller et al. (2023) includes data from aboveground biomass samples (n = 27), soil cores (n = 132) and belowground biomass (roots, stolons, and rhizomes) samples (n = 33) collected from six sites in Scotland ( Figure 1 ) following the same sampling and analytical approaches used in this study. The belowground biomass samples were collected and analyzed following the standard methodology (Harvey et al., 2019; Penk et al., 2020). Biomass samples were loosened by hand, prior to gently shaking for 3 hrs in a 5% solution of sodium hexametaphosphate. The remaining soil was washed from the roots through a 500 µm sieve, which retained any loose root fragments. The fragments were combined with the main portion of belowground biomass, oven dried (60°C, 72 hrs), and weighed so the belowground biomass could be calculated. The dried material was milled to a fine powder and underwent bulk elemental analysis to quantify the belowground C.

Additional aboveground biomass data was acquired from 234 sites within England (Ford et al., 2012; Ford et al., 2016). Of these, 31 sites were rejected for not having the required meta-data (i.e., vegetation classification). Belowground biomass data was obtained from a further 307 sites across England and Wales (Ford et al., 2015; Ford et al., 2019) to supplement the 33 samples from Scotland (Miller et al., 2022b), resulting in a total of 4,797 samples.

To test if the differences in OC content, dry bulk density and soil thickness across the marsh zones and different soil units across GB saltmarshes, ANOVA and Tukey-Kramer (TK) statistical tests (Driscoll, 1996) were utilized.

Progressing from the seaward edge to the landward side of the saltmarsh, the quantity of OC held within the vegetation increased from 0.06 ± 0.06 kg C m^-2 to 0.15 ± 0.15 kg C m^-2 ( Figure 4A ). The invasive Spartina alterniflora and S. anglica species, which often displace native vegetation in the pioneer and low marsh at lower latitudes (Hammond and Cooper, 2002), held 0.11 ± 0.08 kg C m^-2 and outperformed the native pioneer vegetation due to the increase in biomass (clumping) associated with Spartina (Qi and Chmura, 2023). The calculated aboveground OC storage values across all marsh zones are comparable to values (0.09 – 0.28 kg C m^-2) previously observed in GB (Beaumont et al., 2014; ABPmer, 2020; Miller et al., 2023) and other temperate European saltmarshes (Hemminga et al., 1996; Burke et al., 2022; Penk and Perrin, 2022; Carrasco-Barea et al., 2023), but are significantly smaller than values associated with tropical systems (Santini et al., 2019). The ANOVA highlights that the difference between the average aboveground OC storage is statistically significant ( Supplementary Table 5 ), while the TK test reveals the most significant difference to be between the aboveground OC found in the mid-low and pioneer zones ( Supplementary Table 6 ).

The compiled belowground biomass data (n = 340) comprise of observations from the pioneer, mid-low and high saltmarsh zones. Unlike the aboveground data, there are no values available for marsh colonized by Spartina ( Figure 4B ). In contrast to aboveground data, the belowground biomass showed no significant difference between the marsh zones ( Supplementary Table 7, 8 ). The low amount of data from the pioneer zone likely does not fully reflect the range of belowground OC values, leading to the small differences observed between the zones.

The belowground OC data compiled in this study ( Figure 4B ) cover a greater range than observations (0.82 – 1.65 kg C m^-2) previously used in OC stock estimates for GB saltmarshes (Beaumont et al., 2014; Ford et al., 2019; Miller et al., 2023). Nevertheless, they are comparable to values from temperate European saltmarshes which range between 0.22 to 3.75 kg C m^-2 (Van de Broek et al., 2018; Burke et al., 2022; Graversen et al., 2022).

Across the 26 sampled saltmarshes, the thickness of the different soil units (section 3.6.3.1) differed significantly both within and between saltmarshes ( Figure 5 ), as highlighted by the ANOVA and TK tests ( Supplementary Table 9, 10 ). The saltmarsh soil thickness ranges from 57 ± 16 cm in the Kyle of Tongue in north Scotland to 22.93 ± 9.58 cm at Newton marsh in southern England ( Figure 1 ). On average, the saltmarsh soil (fibrous peat, humified peat, transitional soil units) thickness within GB marshes is 28.19 ± 16.32 cm. Unlike the individual soil units, the statistical test show that the thickness of the saltmarsh soil (i.e., the fibrous peat, humified peat and the transitional units) is does not vary significantly between marshes ( Supplementary Table 11, 12 ). The basal unit which represents the pre-saltmarsh environment (e.g., intertidal flat) therefore makes up a significant proportion of the upper 1 m of soil.

Patterns of relatively thin saltmarsh soils overlaying marine sediments are not unique to GB. Such patterns have also been observed in the Wadden Sea (Mueller et al., 2019) and are a product of regional changes in Holocene relative sea level (Shennan et al., 2018) as well as local drivers, including sediment supply and coastal management practices.

The dry bulk density of the soil units differs across the 26 saltmarshes ( Figure 6B ) with values ranging between 0.10 g cm^-3 in the fibrous peat layer at the Kyle of Tongue to 1.68 g cm^-3 in the clay-rich basal unit at Black Rock marsh. Within the studied saltmarshes, the average dry bulk density of the saltmarsh soil is 0.55 ± 0.32 g cm^-3, while the observed basal unit value is 0.84 ± 0.43 g cm^-3. The dry bulk density increases down the soil profile ( Figure 3 ) from the loosely consolidated fibrous peat unit to the more homogenous humified peat and transitional soil units, with the basal unit at each saltmarsh consistently having the highest dry bulk density values ( Figure 5B ). Within each saltmarsh, the high, mid-low, and pioneer marsh zones all exhibit similar dry bulk density values for each of the four soil units ( Supplementary Data ) indicating the dry bulk density is primarily driven by a suite of processes including: (i) the quantity ( Figure 5A ) and porosity of the organic matter (ii) the level of natural soil compaction and (iii) the dominant sediment type (sand vs mud) of the surrounding environment.

The OC content of the soil units differs significantly between marshes with OC values ranging from below 0.1% in the basal units of marshes dominated by sand (i.e., Caerlaverock, Llanrhidian, Shell Island, Sunderland) to over 40% in the fibrous peat layers in the marshes of Orkney (i.e., Bridge of Waithe and Waulkmill Bay) which are fed by catchments containing blanket peat (Porter et al., 2020). Across the 26 saltmarshes the average OC content of the saltmarsh soil and basal unit are 7.94 ± 6.86% and 2.96 ± 2.92% respectively. The high marsh zone soils have the greatest OC content, with a decrease observed in a seaward direction ( Figure 6 ). The ANOVA and TK analysis indicates that the dry bulk density and OC content of the high and mid-low marsh zones are statistically similar, as are the pioneer and Spartina zones ( Supplementary Table 19, 20 ). In contrast, the difference in dry bulk density and OC content of high and mid-low zones is statistically different to that found in the pioneer and Spartina zones across GB ( Supplementary Table 19, 20 ).

Zone-specific vegetation composition is likely the primary driver of the differences observed in OC. Within saltmarshes, it is well understood that the OC content of the soil is driven by the vegetation either through direct OC input from the roots and dead biomass or by the vegetation structures (including leaves, stems, roots, stolons, and rhizomes), facilitating the capture of allochthonous OC (Ford et al., 2019; Austin et al., 2021; Penk and Perrin, 2022).

The diverse range of soil dry bulk density and OC content values ( Figure 6 ) measured in this study are comparable with saltmarsh systems in the UK and Western Europe (Beaumont et al., 2014; Van de Broek et al., 2018; Burden et al., 2019; Ford et al., 2019; Harvey et al., 2019; Marley et al., 2019; Mueller et al., 2019; Smeaton et al., 2020; Smeaton et al., 2022; Burke et al., 2022; Ladd et al., 2022; Miller et al., 2023).

The 26 saltmarshes contain vastly different quantities of OC, ranging from 957 ± 484 tonnes at Loch Laich to 197,862 ± 105,116 tonnes at Llanrhidian ( Figure 7A ). Across all sampled saltmarshes, the soils represent between 84% and 99% of the saltmarsh OC stock, with the above and belowground biomass only representing minor components of the total stock ( Figure 7B ). The magnitude of the saltmarsh OC stock is primarily driven by saltmarsh areal extent, with the largest systems holding the most OC. The largest quantities of OC are stored in the large marshes situated on open coastlines such as those in the Wash (Stiffkey, Gedney) and the Solway Firth (Wigtown, Caerlaverock). These systems are all > 5 km² in size, extending to over 20 km² (Gedney); in contrast the small saltmarshes of Scotland hold the smallest quantity of OC ( Figure 7A ).

When the basal unit (i.e., pre-saltmarsh intertidal flat sediments) is considered, the OC stocks increase by between 15% and 77% depending on location. In the saltmarshes of northern Scotland, we observe the smallest increase in OC stock. These saltmarshes generally have soils associated with saltmarsh habitat to a greater depth than the rest of the country ( Supplementary Data ). In addition, the difference in OC content between the saltmarsh soil and the basal unit can be significant ( Figure 6C ). The difference is most pronounced at Morrich More where the saltmarsh soil contains 18.85 ± 13.08% OC in comparison to the sandy basal unit which holds 0.64 ± 0.34% OC. These factors result in the saltmarsh soils in the north of Scotland holding a greater quantity of OC than the pre-saltmarsh sediments. The opposite is true in marshes such as Tyninghame and Stiffkey where the pre-saltmarsh sediments hold significantly more (>75%) OC than the saltmarsh soils. In these marshes, the saltmarshes soils are thinly layered on top of much thicker deposits of pre-saltmarsh sediments. Furthermore, the difference in OC content between the saltmarsh soil and basal unit is far smaller. For example, the saltmarsh soil in the high marsh zone of Gedney contains 5.66 ± 0.85% OC, with the basal unit holding 3.99 ± 1.36% OC. The reduced depth of the saltmarsh soils, combined with the comparable OC contents of the saltmarsh soils and basal unit at these sites, results in the pre-saltmarsh sediment containing the majority of the OC held within the top 1 m. This clearly highlights the importance of understanding the temporal development of the saltmarsh and how this is reflected in the soil profile to assure the accurate quantification saltmarsh OC and to avoid the inclusion of OC held within underlying material deposited prior to saltmarsh development.

While saltmarsh areal extent clearly drives the magnitude of the saltmarsh OC stock, there are clear differences in the effectiveness of how individual saltmarshes store OC ( Figure 7C ; Figure 8 ). When normalized for area, the smaller saltmarshes of Scotland (such as Bridge of Waithe and the Kyle of Tongue) have both the smallest OC stocks yet per are unit store the greatest quantity. In contrast, the saltmarshes with the largest stocks (such as Llanrhidian and Stiffkey) store much smaller quantities per unit area ( Figure 8 ). The differences are likely driven by regional and local factors such as geomorphology, source of the OC, sediment type (mud vs sand) and sediment supply (Kelleway et al., 2016).

The saltmarshes in the north and northeast of Scotland (Bridge of Waithe, Kyle of Tongue, Cambusmore Lodge, Waulkmill Bay, Morrich More) store between 23.97 and 40.51 kg C m^-2 ( Figure 8 ), which far exceeds the global average of 16.2 kg C m^-2 (Duarte et al., 2013) and that observed in other temperate saltmarshes in northwest Europe (Burke et al., 2022; Graversen et al., 2022). The above average OC storage values for these systems are likely attributed to allochthonous input from the OC rich terrestrial environment. The catchments of these saltmarshes are dominated by peatlands and represent some of the most OC rich environments in Europe (Lilly and Donnelly, 2012). Recent work highlighted that up to 89.1 ± 12.1% of the OC held within northern Scottish saltmarshes originates from the terrestrial/in situ sources (Miller et al., 2023). Additionally, regional differences in Holocene relative sea level history (Shennan et al., 2018; Bradley et al., 2023) across GB has resulted in a greater period of stability and time for saltmarsh soils to accumulate in north Scotland. For example, while the saltmarsh at the Kyle of Tongue began to develop around 2,000 years ago (Barlow et al., 2014), Newton Marsh on the Isle of Wight only began to form 300 years ago (Long et al., 2014). The combination of these factors likely explains why the northern Scottish saltmarshes store the greatest quantity of OC per area unit of any European saltmarshes to date.

A second group of saltmarshes (Tay, Skinflats, Dornoch Point, Forth (Alloa)) located in the estuaries of major rivers of Scotland also ranks above average in terms of OC storage, with values ranging between 10.36 and 27.71 kg C m^-2. Again, allochthonous OC input is the most likely driver of the elevated OC storage values; together the rivers Tay and Forth drain 6,023 km² of the OC-rich soils of mainland Scotland. Miller et al. (2023) found that 93.2 ± 14.5% of the OC in the saltmarsh soils of Skinflats originates from terrestrial/in situ sources.

The remaining 17 saltmarshes store similar quantities of OC with values ranging between 2.24 and 11.67 kg C m^-2, with an average value from across these marshes of 8.03 ± 2.62 kg C m^-2 ( Figure 8 ). These saltmarshes fall below the global average of 16 - 40 kg C m^-2 (Duarte et al., 2013; Temmink et al., 2022), yet are comparable to values found in both the Republic of Ireland (6.46 – 18.58 kg C m^-2) and Denmark (4.27 – 8.18 kg C m^-2) (Burke et al., 2022; Graversen et al., 2022). The lower OC storage values of the 17 saltmarshes in comparison to the Scottish systems is likely due to their catchments having less OC to supply marsh soils (Bradley et al., 2005). On average, the soils of these marshes 17 contained 6.34 ± 3.66% OC, compared to the 11.67 ± 13.05% OC of the Scottish saltmarsh soils. Additionally, the more southern saltmarshes have also had significantly less time to develop due to regional relative sea level history. Within this group of 17 saltmarshes, the variance in OC storage is largely driven by the dominant sediment type of the surrounding environment. The lowest OC storage values are found in saltmarshes such as Sunderland, Black Rock and Tyninghame which are sand-dominated and only contain a store a small quantity of OC ( Figure 8 ), whereas the mud rich saltmarshes (Newton, Arne, Lindisfarne) contain higher quantities of OC and store more OC per area unit ( Figure 8 ).

A combination of several regional and local factors likely govern the quantity of OC stored within saltmarshes around GB. Here we have identified the drivers that provide first-order control on OC storage (OC source, relative sea-level history, and sediment type). Further work is required to fully understand the interaction between first-order controls and other factors such as sediment supply and climatic conditions on OC storage.

Globally, saltmarshes are threatened by historical losses, increased anthropogenic disturbance, and a rapidly changing climate (Pendleton et al., 2012; Barbier, 2013; Campbell et al., 2022). Quantifying the OC stored in a nation’s saltmarsh habitat is a foundational step towards the development of nation-specific climate policy and management interventions to protect and preserve these at-risk costal ecosystems. For example, the third UK Climate Change Risk Assessment (CCRA3) states it is crucial to protect our natural C stores from climate and anthropogenic related threats in order for the UK to meet net zero commitments (Betts et al., 2021). To achieve this ambition, the CCRA3 underlines the urgent requirement for a baseline assessment of the total C stocks in coastal habitats to quantify the potential impact on climate from habitat loss and to assess the success of future management interventions (i.e., OC gains or losses) (Betts et al., 2021). The UK Blue Carbon Evidence Partnership (UKBCEP) echoes these points, asserting that a stronger evidence base including C stock assessments will enable more accurate Greenhouse Gas emissions reporting (GHG) and accounting of the UK’s natural capital (UKBCEP, 2023).

This study achieves the ambition of the CCRA3 and the UKBCEP and provides a baseline assessment of saltmarsh OC stocks for UK saltmarshes. From this assessment, we can identify hotspots for saltmarsh OC storage and quantify the potential climate risks from disturbance of the ecosystem, thereby allowing the prioritization of management interventions to protect and preserve these natural C stores.